"How much more?"
I tried "כמה יותר" because I've said this and no one corrected me. What is the difference between יותר and עוד?
You can use יותר only to compare between two amounts. (e.g יש יותר נשים מגברים בחדר). In this sentence you are not comparing anything, therefore "כמה יותר" would be incorrect.
as a question ("how much more") it's grammatically incorrect, but if it is a part of request ("can I take a few more strawberries?" - "אני יכולה לקחת עוד כמה תותים?") it is possible and grammatically correct